1. Field of the Invention
The present invention relates to a road gradient detecting device applicable to an automotive vehicle in which wheels are driven by an engine via a transmission device having a starter clutch interposed therein and a starter clutch controlling device using the road gradient detecting device.
2. Description of the Related Art
Conventionally known as a starter clutch controlling device is a starter clutch controlling device in which a starter clutch is brought into engagement even under a specific running condition where an engine is in an idling state and the vehicle speed is equal to or slower than a predetermined value and in which the engagement force of the starter clutch while an automotive vehicle is at a stop with a brake pedal being depressed is reduced lower than when the brake pedal is not depressed in order to eliminate vibrations of a vehicle body and deterioration in fuel economy which both result from a creep torque while brakes are in operation (refer to JP-A-62-216842 and JP-A-1-244930).
In this starter clutch controlling device, when the brakes are released, the engagement force of the starter clutch increases and the creep torque is restored to an original magnitude, but the engagement force of the starter clutch does not increase instantaneously, and there occurs a slight time lag before the creep torque is restored to the original magnitude. Here, when trying to start a vehicle from an ascending slope, if there occurs a time lag as described above in restoration of the creep torque to the original magnitude after the release of the brakes, there may occur a case in which the driver has to depress the accelerator pedal after he or she releases the brakes in order to obtain a torque counteracting against a reverse torque generated by the gravity of the vehicle.
Then, in order to improve the operability at the time of starting off the vehicle., there is known a starter clutch controlling device in which a means for detecting the gradient of a road is provided which is adapted to prohibit the engagement force of a starter clutch provided when the brakes are applied from decreasing when the gradient of a road is an ascending gradient of a predetermined value or greater (JP-A-9-210093).
In addition, JP-A-9-207735 discloses a road gradient detecting device for controlling shifting of an automatic transmission in which the gradient of a road is calculated from a wheel driving force and acceleration of the vehicle.
Although it is conceived that the above road gradient detecting device is used as a road gradient detecting means for a starter clutch without using any specific sensor, the detecting device calculates a wheel driving force based on an output torque of the engine, and the device cannot accurately calculate a wheel driving force in a state in which the starter clutch slips when the vehicle runs at such low speed as it is easily brought to a stop or when the vehicle slows down, thus making it impossible to secure the required road gradient calculating accuracy. Due to this, there may be caused a risk that the control of the starter clutch in response to the gradient of a road where the vehicle is stopped cannot be performed properly.
The present invention was made in view of the above situation, and a first object of the invention is to provide a road gradient detecting device adapted to accurately calculate a road gradient even in a state in which a starter clutch slips. A second object of the invention is to provide a starter clutch controlling device adapted to properly perform the control of a starter clutch in response to the gradient of a road where the vehicle is stopped.
With a view to solving the first object, according to a first aspect of the invention, there is provided a road gradient detecting device applicable to an automotive vehicle in which wheels are driven by an engine via a transmission device having a starter clutch interposed therein, wherein a gradient of a road is calculated from a wheel driving force and vehicle acceleration, the road gradient detecting device comprising a first driving force calculating means for calculating a wheel driving force based on an output torque of the engine, and a second driving force calculating means for calculating a driving force based on a torque transmitted by the starter clutch which is obtained from an engagement force of the starter clutch, the wheel driving force calculated by the second driving force calculating means being used for calculating a road gradient in a state in which the starter clips.
In addition, with a view to solving the second object, according to the invention, there is provided a starter clutch controlling device for controlling the engagement force of a starter clutch interposed in a transmission device for transmitting an output torque of an engine to wheels, comprising a first controlling means for reducing the engagement force of the starter clutch when an automotive vehicle is at a stop with a brake pedal being depressed lower than when the brake pedal is not depressed, a road gradient detecting means for detecting the gradient of a road, and a second controlling means for increasing the engagement force of the starter clutch when the automotive vehicle is at a stop with the brake pedal being depressed to a value greater than a value obtained by the first controlling means in a case where the gradient of a road is an ascending gradient of a predetermined value or greater, wherein the road gradient detecting device as set forth in the first aspect of the invention is used as the road gradient detecting means.
When the starter clutch slips, a torque exceeding a transmitting torque of the starter clutch cannot be transmitted to wheels, and the wheel driving force becomes a torque corresponding to the transmittable torque of the starter clutch. However, according to the road gradient detecting device of the invention, in the state in which the starter clutch slips, a wheel driving force for use in calculation of the gradient of a road is calculated based on the transmittable torque of the starter clutch, and therefore even if the starter clutch slips, a wheel driving force can be calculated accurately to thereby secure a required accuracy at which a road gradient is calculated.
In addition, according the starter clutch controlling device of the invention, while the vehicle is stopped with the brake pedal being depressed, if the gradient of a road is an ascending gradient of a predetermined value or greater, the gradient can securely be detected and the engagement force of the starter clutch can be controlled to be a value provided by the second controlling means. Thus, the control of the starter clutch can properly be performed in response to the gradient of a road where the vehicle is stopped.
Note that in an embodiment that will be described later, what corresponds to the first driving force calculating means is a step designated as S21-6 in FIG. 6, and what corresponds to the second driving force calculating means is a step designated as S21-12 in FIG. 6, what corresponds to the first controlling means is a step designated as S8 in FIG. 3, and what corresponds to the second controlling means is a step designated as S9 in FIG. 3.